Beam shaping and projection imaging with solid state UV gaussian beam to form vias

ABSTRACT

A diode-pumped, solid-state laser ( 52 ) of a laser system ( 50 ) provides ultraviolet Gaussian output ( 54 ) that is converted by a diffractive optical element ( 90 ) into shaped output ( 94 ) having a uniform irradiance profile. A high percentage of the shaped output ( 94 ) is focused through an aperture of a mask ( 98 ) to provide imaged to provide imaged shaped output ( 118 ). The laser system ( 50 ) facilitates a method for increasing the throughput of a via drilling process over that available with an analogous clipped Gaussian laser system. This method is particularly advantageous for drilling blind vias ( 20   b ) that have better edge, bottom, and taper qualities than those produced by a clipped Gaussian laser system. An alternative laser system ( 150 ) employs a pair of beam diverting galvanometer mirrors ( 152, 154 ) that directs the Gaussian output around a shaped imaging system ( 70 ) that includes a diffractive optical element ( 90 ) and a mask ( 98 ). Laser system ( 150 ) provides a user with the option of using either a Gaussian output or an imaged shaped output ( 118 ).

RELATED APPLICATIONS

[0001] This patent application derives priority from U.S. Pat. Appl.Ser. No. 10/188,282, filed Jul. 1, 2002, which derives priority derivespriority from U.S. Pat. Appl. Ser. No. 09/580,396, filed May 26, 2000,which derives priority from U.S. Provisional Application No. 60/193,668,filed Mar. 31, 2000, from U.S. Provisional Application No. 60/175,098,filed Jan. 7, 2000, and from U.S. Provisional Application No.60/136,568, filed May 28, 1999.

COPYRIGHT NOTICE

[0002] © 2001 Electro Scientific Industries, Inc. A portion of thedisclosure of this patent document contains material which is subject tocopyright protection. The copyright owner has no objection to thefacsimile reproduction by anyone of the patent document or the patentdisclosure, as it appears in the Patent and Trademark Office patent fileor records, but otherwise reserves all copyright rights whatsoever. 37CFR §1.71(d).

TECHNICAL FIELD

[0003] The invention relates to a diode-pumped solid-state laser and, inparticular, to employing such a laser to generate an ultraviolet laserbeam having a TEM₀₀ non-astigmatic spatial mode to drill vias.

BACKGROUND OF THE INVENTION

[0004] U.S. Pat. Nos. 5,593,606 and 5,841,099 of Owen et al. describetechniques and advantages for employing UV laser systems to generatelaser output pulses within advantageous parameters to form through-holeor blind vias through at least two different types of layers inmultilayer devices. These parameters generally include nonexcimer outputpulses having temporal pulse widths of shorter than 100 ns, spot areaswith spot diameters of less than 100 μm, and average intensities orirradiances of greater than 100 mW over the spot areas at repetitionrates of greater than 200 Hz.

[0005] Lasers are described herein only by way of example to ultraviolet(UV) diode-pumped (DP) solid-state (SS) TEM₀₀ lasers that generate anatural Gaussian irradiance profile 10 such as shown in FIG. 1, but thedescription is germane to almost any laser generating Gaussian output.Ablating particular materials with any laser, and particularly a UV DPSSlaser, is contingent upon delivering to a work piece a fluence or energydensity (typically measured in units of J/cm²) above the ablationthreshold of the target material. The laser spot of a raw Gaussian beamcan be made quite small (typically on the order of 10 to 15 μm at the1/e² diameter points) by focusing it with an objective lens.Consequently the fluence of the small focused spot easily exceeds theablation threshold for common electronic packaging materials,particularly the copper typically used in the metallic conductor layers.Hence, the UV DPSS laser, when used in a raw, focused beamconfiguration, is an excellent solution for drilling vias through one ormore copper layers in an electronic packaging work piece. Since thefocused spot is typically smaller than the desired size of the via, thefocused spot is moved in a spiral, concentric circular, or “trepan”pattern to remove sufficient material to obtain the desired via size.This approach is commonly referred to as spiraling or trepanning withthe raw, focused beam. Spiraling, trepanning, and concentric circleprocessing may generically be referred to as “nonpunching” forconvenience.

[0006] An alternative approach that is also well known in the artinvolves passing the TEM₀₀ laser beam with the Gaussian irradianceprofile through a circular aperture or mask of a predetermined diameter12. One or more common refractive optic lenses are then used to projectan image of the illuminated aperture onto the work surface. The size ofthe imaged circular spot depends on both the size of the aperture andoptical de-magnification obtained with the refractive imaging lens orlenses. This technique, known as projection imaging or simply imaging,obtains a desired via diameter by adjusting either the aperture size orthe optical de-magnification or both, until the size of the imaged spotmatches the desired via size. Because the low-intensity “wings” of theGaussian irradiance profile are masked or clipped by the aperture edges,this imaging technique is, therefore, also called clipped-Gaussianimaging.

[0007] When drilling vias with the imaged spot, the laser beam simplydwells at the via site for a number of pulses until sufficient materialhas been removed. This drilling method, often called “punching,”eliminates the extremely precise and fast in-via movement of the laserspot that is required when trepanning or spiraling with the raw, focusedbeam. Thus, via drilling with a clipped Gaussian beam reduces thedemands placed upon the high-speed beam positioner, since it eliminatesthe complex small-radius, curved pathways and attendant highaccelerations associated with inside-the-via motions. Processdevelopment is also simpler with projection imaging because there arefewer process parameters to be optimized.

[0008] Clipped Gaussian processing also produces much rounder and morerepeatable vias because the inherent variations in laser spot roundnessfrom laser to laser no longer govern the shape of the via, rather theroundness is largely determined by the circularity of the aperture andthe quality of the optics used to project its image onto the worksurface. Roundness is also secondarily impacted by throughput and thedegree to which the wings of the raw Gaussian pulse is clipped.Roundness, or circularity, may be quantified as a ratio of minimumdiameter to the maximum diameter typically measured at the top of thevia, i.e. R=d_(min)/d_(max). The rounder spots are possible because onlythe central portion of the Gaussian irradiance profile of the laser beamis permitted to pass through the aperture; hence the low-irradianceouter regions of the Gaussian beam are blocked or clipped by theaperture mask.

[0009] A problem with a clipped Gaussian beam is, however, that itscenter is more brightly illuminated than its edges. This nonuniformityadversely affects the quality of vias created with this beam,particularly blind vias, resulting in vias having rounded bottoms anduneven edges and risking damage to the underlying or neighboringsubstrate.

[0010] A laser system employing the clipped Gaussian technique can beimplemented so that varying fractions of the Gaussian beam are blockedby the aperture. If the Gaussian irradiance profile is highly clipped sothat only a small portion of the output beam center is allowed to passthrough the aperture, then the irradiance profile imaged onto the worksurface will be more nearly uniform. This uniformity comes at theexpense of rejecting a large fraction of the energy at the aperture maskand hence not delivering the energy to the work surface. Wasting suchlarge portions of beam energy impedes drilling speed.

[0011] If, on the other hand, a large fraction of the beam energy ispermitted to pass through the aperture, then higher fluence is deliveredto the work. However, the difference between the irradiance at the spotcenter, I_(c), and the spot edges, I_(e), will be large. The fraction ofenergy passing through the aperture is commonly known as thetransmission level, T. For a Gaussian beam, the following mathematicalrelationship exists:

T=1−I _(e) /I _(c)

[0012] For example, if 70% of the beam energy passes through theaperture, then both the irradiance and the fluence at the edge of theimaged spot will be only 30% of the value at the center of the spot.This difference between I_(c) and I_(e) causes tradeoffs in the drillingprocess.

[0013] If high laser power is used in order to drill more rapidly, thefluence at the spot center, Fc, can exceed the fluence at which thecopper at the via bottom begins to melt and reflow. At the same time, ifT is large (and therefore the edge-to-center fluence ratio Fe/Fc withinthe spot is small), the edges of the imaged spot have low fluence and donot ablate the organic dielectric material rapidly. FIG. 2 is graph ofedge fluence versus aperture diameter for clipped Gaussian output undertypical via processing parameters. As a result, many pulses are requiredto clear the dielectric material (such as an epoxy resin) from the edgesof the via bottom and thereby obtain the desired diameter at the viabottom. Applying these pulses, however, may damage the center of the viadue to the high fluence in that region which melts the bottom copper.

[0014] The clipped Gaussian technique, therefore, forces a trade-offbetween high pulse energy that drills rapidly but damages the center ofthe via bottom and lower pulse energy that is below the copper reflowthreshold fluence but drills slowly and requires many pulses to clearthe via edges. Typically, depending on the via size, transmission levelsbetween 30% and 60% offer an acceptable compromise between wasted(blocked) laser energy and the undesirable process phenomena related tonon-uniformity of the fluence within the imaged spot. Small vias can bedrilled at acceptable speed with lower transmission levels (e.g. 25-30%)and therefore higher uniformity of the imaged spot. However, for manyapplications, 50%<T<60% is desirable to obtain acceptable speed, and viaquality suffers due to bottom copper damage.

[0015] A more energy and speed efficient method for drilling vias istherefore desirable.

SUMMARY OF THE INVENTION

[0016] An object of the present invention is, therefore, to provide amethod and/or system that improves the speed or efficiency of viadrilling with a Gaussian beam while improving the via quality.

[0017] Another object of the invention is to provide such a method orsystem that employs a UV, diode-pumped (DP), solid-state (SS) laser.

[0018] The present invention enhances the projection imaging technique.In one embodiment of the invention, a UV DPSS laser system is equippedwith a diffractive optical element (DOE) to shape the raw laser Gaussianirradiance profile into a “top hat” or predominantly substantiallyuniform irradiance profile. The resulting shaped laser output is thenclipped by an aperture or mask and focused by a scan lens to provide animaged shaped output beam at a target surface. The imaged shaped outputbeam has a substantially uniform intensity laser spot from its center toits edge so that high quality vias can be drilled rapidly without riskof bottom damage.

[0019] Conventional systems that utilize beam shaping, projectionimaging, or diffractive optics employ low brightness non-UV lasers orhighly astigmatic and multi-mode Excimer lasers and have been generallyused in applications other than materials processing.

[0020] In many of these via drilling applications, the spatialuniformity is required to make the process work. Without it, thenon-uniformity of the fluence at the work surfaces typically leads toproblems with over-processing in the center of the focused spot andunder-processing at its edges. In the present invention, the beamshaping does not enable the via drilling process. Rather, it enhances itby making the process faster and more controllable. The inventiontherefore provides the ability to enhance the quality, speed, androbustness of the UV laser via drilling process.

[0021] Although other types of devices have been used to producenear-uniform or “homogenized” beams with excimer lasers for materialsprocessing, such homogenizers do not work with the highly coherent,near-TEM₀₀ spatial mode of a DPSS high-brightness laser. Further, sinceunlike the large spots inherent to an excimer laser beam, the TEM₀₀spatial mode is highly focusable, so the present invention can utilize amuch higher percentage of the incident energy.

[0022] Additional aspects and advantages of this invention will beapparent from the following detailed description of preferredembodiments thereof, which proceeds with reference to the accompanyingdrawings.

BRIEF DESCRIPTION OF THE DRAWINGS

[0023]FIG. 1 is a perspective view of a three-dimensional Gaussianirradiance profile of a typical prior art DPSS laser pulse.

[0024]FIG. 2 is graph of edge fluence versus aperture diameter forclipped Gaussian output under typical via processing parameters.

[0025]FIG. 3 is an enlarged, cross-sectional side view of vias drilledinto a portion of a generic laser work piece.

[0026]FIG. 4A is a simplified side elevation and partly schematic viewof an embodiment of a laser system employed for increasing via drillingthroughput in accordance with the present invention.

[0027]FIG. 4B is a simplified side elevation and partly schematic viewof an alternative embodiment of a laser system employed for increasingvia drilling throughput in accordance with the present invention.

[0028] FIGS. 5A-5C is a sequence of simplified irradiance profiles of alaser beam as it changes through various system components of the lasersystem of FIG. 4.

[0029] FIGS. 6A-6D are exemplary substantially uniform square orcircular irradiance profiles.

[0030] FIGS. 7A-7D are simplified side elevation and partly schematicviews of four respective exemplary embodiments of beam shaping systemsfor varying the size of an image spot.

[0031]FIG. 8 is a simplified partly schematic plan view of alternativelaser system that employs an auxiliary galvanometer mirror pathway topermit use of a raw focused beam.

[0032]FIG. 9 is a graphical comparison of ideal fluence distributions atthe aperture plane for imaged shaped output and clipped Gaussian outputat several typical transmission levels under typical via processingparameters.

[0033]FIG. 10 is a graphical comparison of throughput curves for clippedGaussian and imaged shaped via drilling techniques.

[0034]FIG. 11 is a graph of via taper ratio as a function of worksurface location relative to the nominal image plane.

[0035]FIG. 12 is a graph of via diameter as a function of work surfacelocation relative to the nominal image plane.

[0036]FIG. 13 is a graph of via roundness as a function of work surfacelocation relative to the nominal image plane.

[0037]FIG. 14 is a copy of an electron micrograph of a 75-μm via drilledin 45-μm thick epoxy resin.

[0038]FIG. 15 is a copy of an electron micrograph a 75-μm via drilled in45-μm thick epoxy resin in a 150-μm thick pre-etched copper opening.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

[0039]FIG. 3 is an enlarged, cross-sectional side view of though holevia 20 a and blind via 20 b (generically via 20) machined into a genericlaser work piece 22 that may, for example, be an MCM, circuit board, orsemiconductor microcircuit package. For convenience, work piece 22 isdepicted as having only four layers 24, 26, 28, and 30.

[0040] Layers 24 and 28 may contain, for example, standard metals suchas, aluminum, copper, gold, molybdenum, nickel, palladium, platinum,silver, titanium, tungsten, metal nitrides, or combinations thereof.Conventional metal layers 24 and 28 vary in thickness, which istypically between 9-36 μm, but they may be thinner or as thick as 72 μm.

[0041] Layer 26 may, for example, contain a standard organic dielectricmaterial such as benzocyclobutane (BCB), bismaleimide triazine (BT),cardboard, cyanate esters, epoxies, phenolics, polyimides,polytetrafluorethylene (PTFE), various polymer alloys, or combinationsthereof. Conventional organic dielectric layers 26 vary considerably inthickness, but are typically much thicker than metal layers 24 and 28.An exemplary thickness range for organic dielectric layers 26 is about30-400 μm, but they may be placed in stacks as large as 1.6 mm.

[0042] Layer 26 may also contain a standard reinforcement component or“layer” 30. Layer 30 may be fiber matte or dispersed particles of, forexample, aramid fibers, ceramics, or glass woven or dispersed intoorganic dielectric layer 26. Conventional reinforcement layers 30 aretypically much thinner than organic dielectric layers 26 and may be onthe order of 1-2 μm and perhaps up to 10 μm. Skilled persons willappreciate that reinforcement material may be introduced as powders intothe organic dielectrics. The layers 30 formed by such powderyreinforcement material may be noncontiguous and nonuniform. Skilledpersons will also appreciate that layers 24, 26, and 28 may also beinternally noncontiguous, nonuniform, and nonlevel. Stacks havingseveral layers of metal, dielectric, and reinforcement material may belarger than 2 mm.

[0043] A through-hole via 20 a typically penetrates all layers andmaterials of work piece 22 from its top 42 to its bottom 44. Blind via20 b does not penetrate all layers and/or materials. In FIG. 3 forexample, blind via 20 b stops at and does not penetrate layer 28. Thetaper of a via 20 is commonly discussed in terms of a ratio of itsbottom diameter d_(b) to its top diameter d_(t). A taper ratio of 66% iscurrently an acceptable standard in the industry, and ratios of 67-75%are considered to be very good. The present invention permits a taperratio of greater than 80% at a maximum throughput with no damage tolayer 28, and taper ratios of greater than 95% are possible withoutdamage to layer 28.

[0044] Via diameters typically range from 25-300 μm, but laser system 50(FIG. 4) may produce vias 20 a and 20 b that are as small as about 5-25μm or greater than 1 mm. Vias smaller than 150 μm diameter arepreferably produced by laser punching. Vias larger than 180 μm arepreferably produced by trepanning, concentric circle processing, orspiral processing.

[0045] With reference to FIG. 4A, a preferred embodiment of a lasersystem 50 of the present invention includes Q-switched, diode-pumped(DP), solid-state (SS) UV laser 52 that preferably includes asolid-state lasant such as Nd:YAG, Nd:YLF, Nd:YAP, or Nd:YVO₄, or a YAGcrystal doped with holmium or erbium. Laser 52 preferably providesharmonically generated UV laser pulses or output 54 at a wavelength suchas 355 nm (frequency tripled Nd:YAG), 266 nm (frequency quadrupledNd:YAG), or 213 nm (frequency quintupled Nd:YAG) with primarily a TEM₀₀spatial mode profile.

[0046] Although Gaussian is used to describe the irradiance profile oflaser output 54, skilled persons will appreciate that most lasers 52 donot emit perfect Gaussian output 54 having a value of M²=1. Forconvenience, the term Gaussian is used herein to include profiles whereM² is less than or equal to about 1.4, even though M² values of lessthan 1.3 or 1.2 are preferred. Skilled persons will appreciate thatother wavelengths are available from the other listed lasants. Lasercavity arrangements, harmonic generation, and Q-switch operation are allwell known to persons skilled in the art. Details of one exemplary laser52 are described in detail in U.S. Pat. No. 5,593,606 of Owen et al.

[0047] UV laser pulses 54 may be converted to expanded collimated pulsesor beam output 60 by a variety of well-known optics including beamexpander or upcollimator lens components 56 and 58 (with, for example, a2 x beam expansion factor) that are positioned along beam path 64. Beamoutput 60 is directed through a shaping and imaging system 70 to producecollimated apertured shaped pulses or beam output 72 that is thenpreferably directed by a beam positioning system 74 to target collimatedapertured shaped output 72 through a scan lens 80 (The scan lens is alsoreferred to as a “second imaging,” focusing, cutting, or objectivelens.) to a desired laser target position 82 at the image plane on workpiece 22.

[0048] Beam positioning system 74 preferably includes a translationstage positioner 76 and a fast positioner 78. Translation stagepositioner 76 employs at least two platforms or stages that supports,for example, X, Y, and Z positioning mirrors and permits quick movementbetween target positions 82 on the same or different circuit boards orchip packages. In a preferred embodiment, translation stage positioner76 is a split-axis system where a Y stage supports and moves work piece22, an X stage supports and moves fast positioner 78 and objective lens80, the Z dimension between the X and Y stages is adjustable, and foldmirrors 75 align the beam path 64 through any turns between laser 52 andfast positioner 78. Fast positioner 78 may for example include a pair ofgalvanometer mirrors that can effect unique or duplicative processingoperations based on provided test or design data. These positioners canbe moved independently or coordinated to move together in response topanelized or unpanelized data. Such a preferred beam positioning system74 that can be used for drilling vias 20 is described in detail in U.S.Pat. No. 5,751,585 of Cutler et al.

[0049] A laser controller (not shown) that directs the movement of thebeam positioning components preferably synchronizes the firing of laser52 to the motion of the components of beam positioning system 74 such asdescribed in U.S. Pat. No. 5,453,594 of Konecny for Radiation BeamPosition and Emission Coordination System.

[0050] An example of a preferred laser system 50 that contains many ofthe above-described system components employs a Model 45xx UV laser (355nm) in a Model 5200 laser system or others in its series manufactured byElectro Scientific Industries, Inc. in Portland, Oreg. Persons skilledin the art will appreciate, however, that any other laser type having aGaussian beam intensity profile, other wavelengths such as IR, or otherbeam expansion factors can be employed.

[0051]FIG. 4B is a simplified side elevation and partly schematic viewof an alternative embodiment of laser system 50 employing a variablebeam expander 55 up stream of imaging system 70 and an optional variablezoom beam expander 120 a (later discussed with respect to FIG. 7) thatis positioned downstream of the IOR. For convenience, certain featuresof laser system 50 in FIG. 4B that correspond to those in FIG. 4A havebeen designated with the same reference numbers. FIGS. 4A and 4B maygenerically be referred to as FIG. 4.

[0052] With reference to FIG. 4B, variable beam expander (VBE) 55includes axially movable expansion and collimation lens components 57and 58 whose positions can be adjusted to obtain a beam of desired sizeat beam shaping component 90. Because the irradiance profile 96 ataperture mask 98 depends the diameter of the Gaussian-profile input beam60 at beam shaping component 90 (discussed in detail in connection withFIG. 5), lens components 57 and 58 can be adjusted to better controlprofile 96 at the aperture mask 98. Moreover, VBE 55 permits a higheroptical throughput for all sizes of aperture mask 98 for the followingreason. The diameter of the shaped output 94 at the plane of aperturemask 98 can be optimized by varying the distance Z so that most of thebeam passes through aperture 98, yielding high optical throughput.However, at the same time, this creates undesirable variations in theirradiance profile 96. VBE 55 compensates for these variations inprofile 96 created by varying distance Z. So used in this fashion, VBE55 permits optical throughput at the aperture to be maximized whileavoiding undesirable effects of the procedure used to do so. VBE 55 alsopermits compensation for variations in beam shaping components 90 and/orfor variations in beam size from laser to laser, and permits greaterconsistency between several laser systems 50. The axial positions oflens components 57 and 58 can be manually or automatically adjusted withthe aid of the laser controller.

[0053] FIGS. 5A-5C (collectively FIG. 5) show a sequence of simplifiedirradiance profiles 92, 96, and 102 of a laser beam as it changesthrough various system components of laser system 50. FIGS. 5Ba-5Bc showsimplified irradiance profiles 96 a-96 c of shaped output 94 (94 a, 94b, and 94 c, respectively) as a function of distance Z with respect toZ₀′. Z₀′ is the distance where shaped output 94 has its flattestirradiance profile shown in irradiance profile 96 b.

[0054] With reference again to FIGS. 4 and 5, a preferred embodiment ofshaped imaging system 70 includes one or more beam shaping components 90that convert collimated pulses 60 that have a raw Gaussian irradianceprofile 92 into shaped (and focused) pulses or output 94 b that have anear-uniform “top hat” profile 96 b, or particularly a super-Gaussianirradiance profile, in proximity to an aperture mask 98 downstream ofbeam shaping component 90. FIG. 5Ba shows an exemplary irradianceprofile 96 a where Z<Z₀′, and FIG. 5Bc shows an exemplary irradianceprofile 96 c where Z>Z₀′.

[0055] Beam shaping component 90 is preferably a diffractive opticelement (DOE) that can perform complex beam shaping with high efficiencyand accuracy. Beam shaping component 90 not only transforms the Gaussianirradiance profile of FIG. 5A to the near-uniform irradiance profile ofFIG. 5Bb, but they also focus the shaped output 94 to a determinable orspecified spot size. Both the shaped irradiance profile 94 b and theprescribed spot size are designed to occur at a design distance Z₀ downstream of beam shaping component 90. In a preferred embodiment, Z₀′ isclose to or equal to distance Z₀. Although a single element DOE ispreferred, skilled persons will appreciate that the DOE may includemultiple separate elements such as the phase plate and transformelements disclosed in U.S. Pat. No. 5,864,430 of Dickey et al., whichalso discloses techniques for designing DOEs for the purpose of beamshaping. Suitable DOEs can be manufactured by MEMS Optical, Inc.,Huntsville, Ala.

[0056] FIGS. 6A-6D (collectively FIG. 6) show exemplary substantiallyuniform irradiance profiles produced by a Gaussian beam propagatingthrough a DOE as described in U.S. Pat. No. 5,864,430. FIGS. 6A-6C showsquare irradiance profiles, and FIG. 6D shows a cylindrical irradianceprofile. The irradiance profile of FIG. 6C is “inverted,” showing higherintensity at its edges than toward its center. Skilled persons willappreciate that beam shaping components 90 can be designed to supply avariety of other irradiance profiles that might be useful for specificapplications, and these irradiance profiles typically change as afunction of their distance from Z₀′. Skilled persons will appreciatethat a cylindrical irradiance profile such as shown in FIG. 6D ispreferably employed for circular apertures 98; cuboidal irradianceprofiles would be preferred for square apertures; and the properties ofother beam shaping components 90 could be tailored to the shapes ofother apertures. For many straight forward via drilling applications, aninverted cylindrical irradiance profile would be preferred.

[0057] With reference again to FIGS. 4-6, shaped pulses 94 arepreferably focused and passed through an aperture mask 98 to sharpen theedge of shaped pulses 94. In a preferred embodiment, aperture 98 ispositioned at the “nominal aperture plane” which is preferably locatedat a distance Z from beam shaping component 90 about where Z=Z₀′, Z*, orZ₀. Z* is about the distance that permits a specified desired amount ofenergy of shaped pulse 94 through an aperture 98 of a given desirablediameter d_(ap). Skilled persons will appreciate that in an idealsystem, Z₀=Z₀′=Z*.

[0058] While positioning aperture 98 at distance Z₀ would be preferredfor most applications on a single laser system 50, positioning aperture98 at distance Z* is employed for groups of laser systems 50 to addressoutput variations from laser 52 to laser 52 and beam shaping element 90to beam shaping element 90. Z* is preferred because Z* is more sensitivethan Z₀′ such that adjustment within the tolerance of distance Z* willnot generally deviate the flatness of irradiance profile 96 b to theextent that it significantly adversely affects via quality orthroughput. An advantage of using distance Z* for placement of theaperture is that Z* permits a variety of laser systems 50 havingvariations in Gaussian output 54 from lasers 52 to employ the sameprocess parameters from laser system 52 to laser system 52 for the sameoperations. Thus, employing Z* facilitates consistency in documentation,training, synchronization, and via quality.

[0059] Mask 98 may comprise a UV reflective or UV absorptive material,but is preferably made from a dielectric material such as UV grade fusedsilica or sapphire coated with a multilayer highly UV reflective coatingother UV resistant coating. Mask 98 has a circular aperture with adiameter of d_(ap) to produce a highly circular imaged shaped pulses110. The aperture of mask 98 may optionally be flared outwardly at itslight exiting side. Skilled persons will appreciate, however, thataperture of mask 98 can be square, have other noncircular shapes, oreven be omitted if images of non-circular spots on the surface of workpiece 22 are desirable or acceptable. Diameter of d_(ap) clips the wings100 of shaped pulses 94 to produce an apertured shaped profile 102 thatdecreases the diameter of shaped pulses 94 at the expense of theirtransmitted energy.

[0060] The transmitted apertured shaped pulse or output 110 is thencollected by a “first imaging” or collection lens 112 of focal length f,to produce substantially collimated apertured shaped output 72 that ispropagated through positioning system 74 and then re-imaged by scan lens80 of focal length f₂ to produce (targeted apertured shaped) lasersystem pulses or output 114 directed toward work piece 22, and creatingimaged shaped output 118 of spot size diameter d_(spot) on work piece22. In a preferred embodiment, lenses 80 and 112 comprise the imagingoptics useful for significant separation of lenses 80 and 112; however,a skilled person will appreciate that a single imaging lens module orcomponent could be employed. In a preferred embodiment, f₁=216 mm andf₂=81 mm, though persons skilled in the art will recognize that otherlens combinations and focus lengths could be substituted. Thecombination of the collection lens 112 and the scan lens 80 produces animage of the uniformly illuminated aperture of mask 98 (oruniform-irradiance non-circular spot if mask 98 is not used) at ade-magnification factor M, where M=f₁/f₂=d_(a)p/d_(spot). In a preferredembodiment of a fixed de-magnification system, M=2.66, although skilledperson will appreciate that other de-magnification factors could beused.

[0061] In a preferred embodiment, beam shaping component 90, aperturemask 98, and first imaging lens 112 are mounted along a rail path on aninterchangeable imaging optics rail (IOR). The rail path is adapted tobe positioned collinearly with the beam path 64. In one embodiment, thedistance Z, f₁, and f₂ are conserved to permit manufacturinginterchangeability of these components in the IOR with analogouscomponents with different properties to perform diverse ranges of spotsizes d_(spot). The positioning of beam shaping component 90 can also bevariable so that distance Z can be adjusted within the tolerances of Z*for each combination of beam shaping components 90 and aperturediameters d_(ap). The effective distance between lenses 112 and 80 isvariable. Thus, several IORs with different combinations of IORcomponents can be quickly exchanged to allow processing operations for adiverse range of predetermined spot sizes. These different combinationsare employed so the beam shape or irradiance profile 96 can be adaptedfor each aperture size d_(ap) to maximize the energy per pulse 62 thatpropagates through the aperture and therefore minimize the energyclipped or wasted by the size limit of the aperture. In addition, toenhancing the efficient use of the pulse energy, the adjustablecoordination between the IOR optical components minimizes any mask 98adaptations that might be desirable to make mask 98 able to withstandlaser damage.

[0062] A disadvantage of this embodiment is, however, the large numberof interchangeable IOR optical components desirable for processing arange of useful spot sizes. For example, each beam shaping component 90may, for example, be efficient for only three to four spot sizesd_(spot), and each mask 98 may, for example, be efficient for only onespot size d_(spot). Thus, to cover the most useful range of spot sizesd_(spot) up to 250 μm, for example, a collection of eight beam shapingcomponents 90 and 25 masks 98 might be employed to provide all of thedesirable combinations.

[0063] FIGS. 7A-7D are simplified side elevation and partly schematicviews of four respective exemplary embodiments of shaped imaging systems70 a, 70 b, 70 c, and 70 d (generically, shaped imaging system 70) forvarying the size of an imaged spot that include exemplary zoom beamexpander (ZBE) lens assemblies 120 a, 120 b, 120 c, and 120 d(generically, ZBE assembly 120). With reference to FIGS. 4 and 7, a ZBEassembly 120 (with tight tolerances to maintain beam accuracy) may bepositioned along beam path 64 between aperture mask 98 and work piece22. ZBE assembly 120 preferably includes lenses 124 and 126 thatcollectively function to vary the spot size. In a preferred embodiment,lens 124 includes a zoom element and functions to change themagnification of laser system 50 and lens 126 includes a compensatorelement that functions to collimate the beam and maintain the focusthroughout a zooming operation. These embodiments may employ optionalalignment mirrors 79 shown in phantom.

[0064] In some embodiments, the focal length f₂ is fixed, but the focallength f, is variable and therefore the de-magnification factor, M, andthe spot size d_(spot) are variable, so each beam shaping component 90may for example efficiently accommodate 8-10 spot sizes or acontinuously varying spot size within a specific range of sizes, andeach aperture may also efficiently accommodate 8-10 spot sizes or acontinuously varying spot size within a specific range of sizes. Thus,to cover the range of spot sizes d_(spot) up to about 250 μm, only a fewbeam shaping components 90 and masks 98 would be employed. In apreferred embodiment, as few as three combinations of beam shapingcomponents 90 and masks 98 could be used to cover the entire range withgreater efficiency.

[0065] ZBE assembly 120 permits a 1 μm resolution over the continuousrange of desirable spot sizes and permits very fine tuning, such as forcompensation for different properties of material interactions. Forexample, some materials will exhibit a 50-μm spot after a given laseroperation while other materials will exhibit a 58-μm spot under the sameconditions. ZBE assembly 120 permits, therefore, an easy method formaking consistently-sized vias in different materials.

[0066] ZBE assembly 120 can also be employed to change the fluence bychanging the spot size. For example, numerous vias through one materiallayer could be processed at one spot size and then the spot size couldbe increased to lower the fluence for processing vias through all of adifferent material layer.

[0067] With reference to FIG. 7A, a zoom lens assembly 120 a (with tighttolerances to maintain beam accuracy) is positioned along beam path 64between first imaging lens 112 and scan lens 80. In zoom lens assembly120 b of the embodiment of shaped imaging system 70 b shown in FIG. 7B,lens 80 and lens 128 are combined into a single lens 130 in zoom lensassembly 120 b. In zoom lens assembly 120 c of the embodiment of shapedimaging system 70 c shown in FIG. 7C, lens 112 and lens 122 are combinedinto a single lens 132 in zoom lens assembly 120 c. In zoom lensassembly 120 d of the embodiment of shaped imaging system 70 d shown inFIG. 7D, lens 80 and lens 128 are combined into a single lens 130 andlens 112 and lens 122 are combined into a single lens 132. Skilledpersons will appreciate that shaped imaging systems 70 a and 70 c arebest suited for a preferred split-axis translation stage positioner 76,and that shaped imaging systems 70 b and 70 d are best suited for beampositioning systems 74 that do not have a fast positioner 78 such asnon-scanning systems that employ a fixed objective lens 80. Skilledpersons will further appreciate that numerous other variable lenscombinations are possible and could be employed without departing fromthe scope of the invention.

[0068] Although positioning system 74 is shown following ZBE assembly120 along beam path 64, some of its components may be positioned toprecede ZBE assembly 120. For example, some components of translationstage positioner may be positioned upstream of ZBE assembly 120, such assome of mirrors 75; however, fast positioner 78 is preferably positioneddownstream of ZBE assembly 120. Skilled persons will appreciate thatthese shaped imaging systems 70 may be supported by separate IORs or asingle IOR system that permits exchange and repositioning of the opticalcomponents and that the IORs supporting the shaped imaging systems canbe easily removed to permit laser system 50 to provide Gaussian outputfor versatility.

[0069]FIG. 8 is a simplified partly schematic plan view of laser system150 that employs galvanometers 152 and 154 to produce an auxiliarygalvanometer mirror pathway 156 that can be added to laser system 50 ofFIG. 4 to permit switching between collimated apertured shaped output 72and Gaussian output 60. With reference to FIG. 8, beam path 64 a isdirected toward galvanometer mirror 158 that either permits the laseroutput to propagate along beam path 64 b through shaped imaging system70 and by galvanometer mirror 162 or reflects the laser output offmirror 164, through optional collimating lens components 166, off mirror168, off galvanometer mirror 162, and toward work piece 22. Mirrors 164and 168 can preferably be adjusted to compensate for pitch and roll.

[0070] Skilled persons will appreciate that collimating lens components166 can be variable to modify the spatial spot size d_(spot) to suitdifferent applications. Alternatively, for example, shaped imagingsystem 70 can instead be positioned along pathway 156 to implementcollimated apertured shaped output 72 so the raw Gaussian beam 60 wouldpropagate along beam path 64 b. Similarly, a shaped imaging system 70can be employed in both beam path 64 b and pathway 156 with each shapedimaging system 70 having variable or different focal lengths to producedifferent spot sizes d_(spot), such as for quick switching between twodifferent imaged shaped spot sizes. Skilled persons will also appreciatethat laser system 150 could employ the Gaussian output to perform avariety of tasks in addition to the via processing applicationsdiscussed herein. For example, laser system 150 could be used to cutcircuits out of panels at high throughput rates.

[0071] Laser systems 50 and 150 are capable of producing laser systemhaving output 114 having preferred parameters of typical via processingwindows that may include average power densities greater than about 100mW measured over the beam spot area, and preferably greater than 300 mW;spot size diameters or spatial major axes of about 5 μm to about 18 μm,preferably from about 25-150 μm, or greater than 300 μm; and arepetition rate of greater than about 1 kHz, preferably greater thanabout 5 kHz or even higher than 30 kHz; an ultraviolet wavelength,preferably between about 180-355 nm; and temporal pulse widths that areshorter than about 100 ns, and preferably from about 40-90 ns orshorter. The preferred parameters of laser system output 114 areselected in an attempt to circumvent thermal damage to via 20 or itssurroundings. Skilled persons will also appreciate that the spot area oflaser system output 114 is preferably circular, but other simple shapessuch as squares and rectangles may be useful and even complex beamshapes are possible with the proper selection of beam shaping component90 cooperating with a desirable aperture shape in mask 98.

[0072] The above-described processing window has been determined tofacilitate via drilling in a wide variety of metallic, dielectric, andother target materials that exhibit diverse optical absorption and othercharacteristics in response to ultraviolet light. Whether punching ornonpunching to create blind vias 20 b, the metal layer is removed with afirst laser output having a power density sufficient to ablate themetal. Then, the dielectric layer is removed with a second laser outputhaving a lower power density that is insufficient to ablate the metal,so only the dielectric is removed and the underlying metallic layer isnot damaged. Thus, the two-step machining method provides a depthwiseself-limiting blind via because the second laser power output isinsufficient to vaporize the metallic bottom layer, even if the secondlaser power output continues after the dielectric material is completelypenetrated.

[0073] Skilled persons will appreciate that in accordance with apunching process of the present invention, the first and second laseroutputs are preferably sequentially contiguous rather than employing aseries of first laser outputs one at a time to spatially separatedtarget positions 82 or work piece 22 and then employing a series ofsecond laser outputs sequentially over the same targets 82. For anonpunching process, layers 24 of all of the spatially separated targetpositions 82 on work piece 22 may be processed with the first laseroutputs before the layers 26 of all of the spatially separated targetpositions 82.

[0074] With reference again to FIGS. 3 and 4, one difference between theclipped Gaussian output of the prior art and imaged shaped output 118 ofthe present invention is that pulse 94 uniformly illuminates theaperture of mask 98 at all points. The imaged shaped output 118consequently facilitates formation of blind vias 20 b with a very flatand uniform bottom 44 b at layer 28 in addition to a very round shapeand crisp edges, and this flatness and uniformity are not possible witha clipped Gaussian beam. In addition, the drilling speed can beincreased with imaged shaped output 118 over that obtainable with aclipped Gaussian beam.

[0075] The addition of a beam shaping component 90 to flatten theirradiance profile 10 of a Gaussian beam minimizes the previouslydiscussed processing tradeoffs between via quality and drilling speedinherent to the clipped Gaussian technique. A high fraction of the beamenergy can be delivered to work piece 22 without a large difference influence between the center and edges of the imaged spot, i.e. theedge-to-center fluence ratio Fe/Fc can be increased while transmissionlevel T is also increased. The present invention permits aperturedshaped output 110 and imaged shaped output 118 to have transmissionlevels of 70-85% without a significant decrease in center to edgefluence ratio.

[0076] As a result of the near-uniform fluence at high transmissionlevels, the drilling speed can be increased without damaging conductorlayer 28, particularly at its center, for two reasons. First, thetransmission level through the aperture is higher than for the clippedGaussian, so more energy is delivered to the work piece 22 in each laserpulse 114. Second, since a higher fluence can be applied to the edges ofthe spot, the dielectric material can be cleared from the bottom edgesof the via more rapidly. This second effect is the more significant ofthe two.

[0077]FIG. 9 shows a comparison of ideal fluence profiles at theaperture plane for shaped output 94 b and clipped Gaussian output atseveral typical transmission levels under typical via processingparameters. Fluence levels on the work piece 22 are equal to theaperture fluence levels multiplied by the imaging de-magnificationfactor squared, which in a preferred embodiment is about a factor ofseven. The fluences at the aperture edge are about 1.05 J/cm² and 0.60J/cm² or less for shaped output 94 b and clipped Gaussian output,respectively. Thus, at work piece 22, the fluences at the edge of theimaged spot are about 7.4 and 4.3 J/cm² for the imaged shaped output 118and clipped Gaussian output, respectively. The rate at which a typicalorganic dielectric material of layer 26 can be ablated differssignificantly between these two fluence levels. As a result, drilling ofeach via 20 can be completed in fewer pulses with the imaged shapedoutput 118, increasing the process throughput.

[0078] An example of a strategy for drilling vias 20 with imaged shapedoutput 118 in accordance with these considerations of present inventionis described below. The fluence across the entire imaged spot can bemaintained, for example, at 90% of the value at which copper damageoccurs, F_(damage). The dielectric material is then ablated atconditions which will not damage via bottom 44 b. In contrast, with theclipped Gaussian beam at T=50%, one could maintain the center of thespot at this fluence, in which case the edges would be at only 45% ofF_(damage). Alternatively, the spot edge could be held at 90% ofF_(damage), in which case the center would be at 180% of the damagethreshold fluence, resulting in substantial damage. Maintaining theedges of the imaged spot at high fluence enables the dielectric materialto be cleared from the via edges with fewer laser pulses, since eachpulse removes more material. Thus, the drilling throughput of imagedshaped output 118 can be much greater than that of the clipped Gaussianoutput.

[0079]FIG. 10 shows a comparison of the throughput curves achieved bythe imaged shaped output 118 and the clipped Gaussian output forpunching 75 μm-diameter vias 20 in 45 μm-deep epoxy resin. Withreference to FIG. 10, the minimum number of pulses, N, necessary toachieve a bottom diameter d_(b) at least 75% as large as the topdiameter d_(t) at each pulse repetition frequency (PRF) was determined.The drilling time was calculated for this value of N at the PRF, and a1.0 ms via-to-via move time was added to obtain the throughput.

[0080] In general, as the laser PRF increases, the energy in each pulse,and therefore the work-surface fluence, steadily decreases. Sincedecreased fluence means less material is removed per pulse, more pulsesmust be applied. However, as the PRF increases, more pulses aredelivered per unit time. The net result is that of two competingeffects, one of which tends to decrease drilling speed and the other ofwhich tends to increase drilling speed with increasing PRF. FIG. 10shows that the competing effects yield the fastest throughputs at PRFsin the middle of the range tested.

[0081]FIG. 10 also shows that the throughput curve achieved with imagedshaped output 118 is flatter than that obtained with clipped Gaussianoutput. The flatter throughput curve is significant for managing thetradeoff between drilling speed and via quality. In order to avoiddamage to the bottom metallic layer 28, it is generally desirable toincrease the laser PRF, thereby decreasing the energy in each pulse andreducing the work-surface fluence below the energy threshold for meltingmetallic layer 28. As the PRF is increased, the throughput obtained withthe imaged shaped output 118 decreases more slowly than that of theclipped Gaussian output. So as the PRF is increased in order maintainvia bottom quality, less of a throughput penalty is incurred with theimaged shaped output 118.

[0082] With reference again to FIG. 10, the imaged shaped output 118enables the peak drilling throughput to be increased over that of theclipped Gaussian by more than 25%. The imaged shaped output 118 alsoachieves higher throughput than is achieved with a raw focused Gaussianbeam, with the added benefits of better via quality (repeatability,sidewall taper, roundness).

[0083] With respect to via quality, particularly for blind vias 20 b,the imaged shaped output 118 of the present invention also permitsbetter taper minimizing performance at higher throughput rates than thatavailable with clipped Gaussian output. In addition to being able toclean dielectric material from the bottom edges of blind via 20 b fasteras discussed above, the imaged shaped output can also clean thedielectric material from the bottom edges of via 20 b more completelywithout risking damage to the underlying metallic layer 28 because theuniform shape of pulse 94 virtually eliminates the possibility ofcreating a hot spot in layer 28 at the bottom center of via 20 b. Theimaged shaped laser output 118 at an appropriate fluence can dwell in ablind via hole indefinitely until a desired cleanliness and taper isachieved.

[0084] Moreover, beam shaping components 90 can be selected to producepulses having an inverted irradiance profile shown in FIG. 6C that isclipped outside dashed lines 180 to facilitate dielectric removal alongthe outer edges of via 20 b and thereby further improve taper. Thepresent invention permits a taper ratio of greater than 80% at a maximumthroughput with no damage to layer 28, and taper ratios of greater than95% (for low aspect ratio vias 20) are possible without damage to layer28. Better than 75% taper ratios are even possible for the smallestvias, from about 5-18 μm diameter at the via top with conventionaloptics although throughput might be diminished.

[0085]FIG. 11 shows the ratio of via bottom diameter to the via topdiameter (62 μm vias drilled in 35 μm particulate-reinforced epoxyresin) as a function work surface location relative to the nominal imageplane, z=0. With reference to FIG. 11, the nominal image plane is thelocation where the vias 20 are most circular, with the most sharplydefined top edges. Positive values of z represent planes below thenominal image plane, i.e., with the work piece 22 placed farther fromthe system optics than distance of separation where z=0. The 3σ errorbar is shown for reference because bottom diameter measurements may bedifficult to measure reliably.

[0086] One hundred vias were drilled and measured at each of nine valuesof z. The data points represent mean values and the vertical error barsrepresent the magnitude of three standard deviations from the mean overeach 100-sample data set. The largest bottom/top ratio is achieved atthe image plane where z=0. Throughout a ±400 μm range, the bottom/topratio was always greater than 75% at high throughput.

[0087]FIG. 12 shows via diameter (in 62-μm vias drilled in 35μm-particulate reinforced epoxy resin) as a function of work surfacelocation relative to the nominal image plane, where z=0. As the workpiece 22 is moved further above the nominal image plane, the average viatop diameter increases steadily. For locations below z=0, the topdiameter remains fairly constant out to 400 μm below the image plane.The 3σ diameters are generally held to within ±3 μm of the averagevalue, with exceptions at z=+300 μm and z=−300 μm. For the bottomdiameters, in contrast, the average value decreases steadily fromlocations above to locations below the nominal image plane. Because thediameter and circularity of the via bottom are significantly moredifficult to control than the size and roundness of the via top, thebottom diameter is shown for reference only. Statistical process controltechniques that could be applied to laser systems 50 and 150 are,therefore, applicable to the characteristics of the via tops.

[0088] The data in FIGS. 11 and 12 suggest several approaches tomanaging depth of focus issues for process robustness. If one wishes tomaintain a constant via top diameter over varying material thicknessesand machine conditions, it would be advantageous to set up the processwith the work surface located slightly below the nominal image plane at,say z=+200 μm. This would produce a zone of ±200 μm of z variation thatcould be accommodated with very little effect on the top diameter. If,on the other hand, it is more desirable to maintain a constant viabottom/top diameter ratio, it would be better to set up the process withwork piece 22 located exactly at the nominal image plane. This wouldensure that the bottom/top ratio would decrease by no more than 5% overa z range of at least ±200 μm. The viability of either of theseapproaches depends on whether the other via characteristics remainwithin acceptable limits as work piece 22 moves away from the nominalimage plane.

[0089] Another issue is via circularity, which is shown in FIG. 13 as afunction of z for 62-μm vias drilled in 35-μm particulate-reinforcedepoxy resin. With reference to FIG. 13, the bottom circularity data havebeen displaced to the right of the actual z values for clarity ofpresentation. The via bottom data are for reference only.

[0090]FIG. 13 shows that the circularity, defined as the minoraxis/major axis, is always at least 90% over the full ±400 μm z range ofthe study. For a 62-μm average diameter, 90% circularity corresponds toa major diameter that is about 6.5 μm larger than the minor diameter.However, for positive z values (locations below the nominal imageplane), the statistical via-to-via variation in circularity becomeappreciable. The error bars shown above the data points (average values)are meaningless above 100% circularity, but at, for example, z=+300 μm,FIG. 13 shows that the 3σ outliers may have circularity below 80%.

[0091] In general, the imaged shaped output 118 of present inventionpermits vias 20 to have a roundness or circularity of greater than 90%at higher throughput rates than achievable with clipped Gaussian output.In many cases, imaged shaped output 118 can achieve a roundness ofgreater than even 95% over the entire range of via sizes at higherthroughput rates.

[0092] Although some of the examples described herein address some ofthe maximum output and other factors involving the use of currentlyavailable UV DPSS lasers 52, skilled persons will appreciate that asmore powerful UV DPSS lasers 52 become available, the via diameters andlayer thicknesses in these examples can be increased.

[0093] Despite the advantages of imaged shaped output 118, projectionimaging may spread the available energy in each imaged shaped laserpulse 118 over a larger area than that typically covered by the ablativeportion of a focused raw Gaussian beam. As a result, UV DPSS lasers 52have energy per pulse limits to the size and thickness of metalliclayers 24 and 28 where the laser spot will exceed the ablation thresholdfluence for the work piece materials.

[0094] With respect to blind vias, for example, imaged shaped pulses 118with fluences of 10-12 J/cm² may be employed to ablate a top copperlayer 24 of 5-12 μm thick for small vias up to perhaps 40 μm indiameter. Skilled persons will appreciate that this fluence rangeimplies a fairly slow repetition rate of about 3-6 kHz, for example.Skilled persons will also appreciate that higher fluences may inviteadverse consequences such as heating, and the resulting slowerrepetition rate would negatively impact throughput. As the powerobtainable with UV DPSS lasers 52 continues to increase, higher-energypulses will be available which will extend the shaped imaging techniqueto through-copper applications for larger via sizes.

[0095] In the interim, a preferred method for punching the top metalliclayer 24 of blind vias 20 b having diameters greater than about 35 μmemploys laser system 150 of FIG. 8. The galvanometer mirror pathway 156is employed to provide focused raw Gaussian output as laser systemoutput 114. The focused raw Gaussian output is used to penetrate the topmetallic layer 24, typically using a nonpunching technique, and then thegalvanometer mirrors 158 and 162 are controlled to allow the laseroutput 60 to pass through imaging system 70 for processing dielectriclayer 26. Skilled persons will appreciate that other types of faststeering mirror methods are well known and could be employed.

[0096] Regardless of how top metallic layer 24 is processed (or it mayeven be pre-etched), the underlying dielectric layer 26 can subsequentlybe machined with imaged shaped output 118 with lower fluences at higherrepetition rates to produce vias 20 with clean round bottoms andnegligible taper as previously described. Typical dielectric processingfluences range from below about 0.8 J/cm², which does little to nodamage to bottom metallic layer 28, to above about 4 J/cm², whichimparts substantial damage to bottom metallic layer 28. Although thepreferred fluence is material dependent, fluences of 1.2-1.8 J/cm² arepreferred for most dielectric layers 26 as the imaged shaped pulses 118approach a copper metallic layer 28.

[0097] Skilled persons will appreciate that there is a throughputadvantage to processing the upper portion of dielectric layer 24 at afluence at the higher end of this range and then reducing the fluence(preferably by increasing the repetition rate) toward the lower end ofthe range as the laser pulses 114 get close to bottom metallic layer 28.For optimum throughput, repetition rates of 12-45 kHz are preferred,12-15 kHz for larger vias 20 b and hard to ablate layers 26 and 30-45kHz for smaller vias. Skilled persons will appreciate that theserepetition rate values will increase as available DPSS laser powerimproves in the future.

[0098] In some applications for medium sized blind vias 20 b, it may bedesirable to use the fast positioner 78 to process the top metalliclayer 24 by nonpunching with the focused Gaussian output and thenpunching through the dielectric with imaged shaped output 118. Skilledpersons will also appreciate that the focused raw Gaussian output oflaser system 150 can also be employed for processing through-hole vias20 a where the via diameters are too large for efficient imaged shapedoutput 118 or where speed is more important than roundness or edgequality.

[0099] With respect to processing the organic or inorganic dielectricmaterials of layer 26, they typically have a much lower ablationthreshold and are easily ablated with a projection imaging configurationup to the largest desirable via diameters. However, for larger via sizesof about 150 μm to about 200 μm and larger, depending on the propertiesof the particular material, the energy distribution of imaged shapedoutput 118 over the via diameter diminishes to a point where thethroughput is adversely affected because each laser system pulse 114removes less material.

[0100] In applications where via diameters exceed about 250-300 μm insize and edge quality and perfect roundness are not as important asthroughput, imaged shaped output 118 or focused Gaussian output of lasersystem 150 is preferably employed to create via 20 by nonpunchprocessing employing fast positioner 78. Skilled persons will appreciatethat nonpunch processing can produce acceptable taper and roundness forlarge vias 20 to suit most applications. This preference applies to boththrough-hole and blind via processing. Skilled persons will alsoappreciate that the imaged shaped output 118 may be more efficient thanthe focused Gaussian output for large size vias in many applications.

[0101]FIG. 14 shows a scanning electron micrograph (SEM) of a typicalvia drilled in epoxy resin with the imaged shaped output 118 of a Uv YAGlaser system 50. The via diameter was 75 μm, the resin thickness was 45μm, and the substrate was prepared by etching away the top copper layerof a resin coated foil on an FR4 core. The bottom (or inner) layercopper was 18 μm (½ oz).

[0102] Several features are noteworthy. First, the via sidewalls areexceptionally smooth and straight, and the top edge of the via issharply defined. Second, owing to image projection of the roundaperture, the via is particularly circular, as previously described.Finally, the bottom copper layer is largely undamaged and free of anyresin residue.

[0103] For this particular test, the beam shaping optics were configuredto produce an inverted fluence profile (FIG. 6C) at the work surfacethat was slightly higher at the spot periphery than in the center. Thelaser parameters (PRF and number of pulses applied) were then adjustedto produce a work-surface fluence at the periphery that was just abovethe value that induces melting of the copper. Close inspection of theimage reveals that the smooth areas near the edges of the via bottom areregions where the copper was lightly reflowed. Such light reflowing ofthe copper may be desired in order to ensure that all resin has beenremoved from the via. This degree of control of the inner layer copperdamage is typical of vias 20 produced by imaged shaped output 118.

[0104] For HDI circuit board microvias, the most common laser drillingtechnique in resin coated foil constructions makes use of a circularopening pre-etched in the top copper layer. This opening is used as aconformal mask for CO₂ laser processing. Layer-to-layer registrationdifficulty has limited this process to drilling of larger vias (>100 μm)with larger pad sizes (>200 μm). However, positioning system 74 permitsthe accurate layer-to-layer alignment of a laser drilling system to becoupled with the higher throughput of drilling only the dielectricresin. In this new process, the outer layer copper is pre-etched to theapproximate size of the inner layer land pad, and the laser is then usedto align and drill a smaller via within this opening. A scanningelectron micrograph of an exemplary via 20 that could be created by thisprocess is presented in FIG. 15, which shows an imaged shaped 75-μm viathrough 45-μm epoxy resin in a 150-μm pre-etched copper opening.

[0105] Skilled persons will appreciate that the beam shaping and imagingtechniques described herein not only permit enhanced via roundness andedge quality, but also permit enhanced repeatability and positioningaccuracy such as in the exact center of pads and may be useful forimproving impedance control and predictability of the electronic workpieces.

[0106] Further comparative data between shaped imaging and clippedGaussian techniques, including color electron micrographs, can be foundin the article entitled “High Quality Microvia Formation with Imaged UVYAG Lasers,” which was presented as a portion of the TechnicalProceedings of the IPC Printed Circuits Expo 2000 in San Diego, Calif.on Apr. 6, 2000.

[0107] It will be obvious to those having skill in the art that manychanges may be made to the details of the above-described embodiments ofthis invention without departing from the underlying principles thereof.The scope of the present invention should, therefore, be determined onlyby the following claims.

1. A laser system, comprising: a diode-pumped, solid-state laser forgenerating ultraviolet Gaussian laser output having a Gaussian energyalong an optical path; a beam-shaping element positioned along theoptical path for converting the Gaussian laser output to shaped outputhaving a central irradiance profile of high and uniform intensity and anouter irradiance profile of low intensity; a variable beam expansionassembly positioned along the optical path between the laser and thebeam-shaping element; an aperture positioned along the optical path forclipping a major portion of the outer irradiance profile of the shapedoutput and passing at least 50% of the Gaussian energy through theaperture to produce apertured shaped output having apertured shapedenergy; one or more imaging lens components for converting the aperturedshaped output into image shaped output; and a positioning system fordirecting the imaged shaped output toward a target location on a workpiece.
 2. The laser system of claim 1 in which the beam-shaping elementcomprises a diffractive optical element.
 3. The laser system of claim 1in which the laser system forms one of a bank of at least twosubstantially similar laser systems for performing the same processingapplication, the laser systems having respective lasers and respectivediffractive optical elements with unintentional performance differences,and in which the variable beam expansion assembly of at least one of thelaser systems to compensate for an unintentional performance differencebetween the lasers or between the diffractive optical elements.
 4. Thelaser system of claim 1 further comprising a zoom lens assemblypositioned along the optical path between the aperture and a targetlocation on a work piece.
 5. The laser system of claim 1 in which theapertured shaped energy is greater than 65% of the Gaussian energy. 6.The laser system of claim 5 in which the apertured shaped energy isgreater than 75% of the Gaussian energy.
 7. The laser system of claim 1in which the wavelength is about 355 nm or about 266 nm.
 8. The lasersystem of claim 1 in which the diffractive optical element is a firstdiffractive optical element, the aperture is a first aperture having afirst size, and the first diffractive optical element and the firstaperture cooperate to produce a substantially uniform first energydensity over a first spot area on the work piece.
 9. The laser system ofclaim 8 in which the first diffractive optical element and the firstaperture are removable and replaceable by a second diffractive opticalelement and a second aperture which cooperate to determine a secondsubstantially uniform energy density over a second spot area, which isdifferent from the first spot area, on the work piece.
 10. The lasersystem of claim 8, further comprising: a first removable imaging opticsrail that houses the first diffractive optical element and the firstaperture, the first imaging optics rail being replaceable with a secondimaging optics rail having a second diffractive optical element and asecond aperture which cooperate to determine a second energy densityover a second spot area, which is different from the first spot area, atthe target location on the work piece.
 11. The laser system of claim 1,further comprising: a removable imaging optics rail that houses thediffractive optical element and the aperture, such that removal of theimaging optics rail permits the Gaussian laser output to impinge thework piece at the target location.
 12. A laser system, comprising: adiode-pumped, solid-state laser for generating ultraviolet Gaussianlaser output having a Gaussian energy along an optical path; abeam-shaping element positioned along the optical path for convertingthe Gaussian laser output to shaped output having a central irradianceprofile of high and uniform intensity and an outer irradiance profile oflow intensity; an aperture positioned along the optical path forclipping a major portion of the outer irradiance profile of the shapedoutput and passing a major portion of the Gaussian energy through theaperture to produce apertured shaped output having apertured shapedenergy; a zoom lens assembly positioned along the optical path betweenthe aperture and a target location on a work piece; one or more imaginglens components for converting the apertured shaped output into imagedshaped output; and a positioning system for directing the imaged shapedoutput toward the target location to form a via.
 13. The laser system ofclaim 12 in which the imaged shaped output has a spot size at the targetin a spot size range between about 10 and 250 μm and in which the zoomlens assembly facilitates spot sizes substantially throughout the spotsize range with resolution of about 1 μm.
 14. The laser system of claim12 in which first and second target materials have different laserablation characteristics such that a given set of laser parameters wouldform a first via in the first target material of a first major spatialdimension and the same set of laser parameters would form a second viain the second target material of a second major spatial dimension andsuch that manipulation of the zoom lens assembly facilitates theformation of a third via in the second target material of the firstmajor spatial dimension with a substantially similar set of laserparameters.
 15. The laser system of claim 12 in which the wavelength isabout 355 nm or about 266 nm.
 16. The laser system of claim 12 in whichthe target location comprises a metal material layer and a dielectricmaterial layer and the zoom lens assembly provides a smaller spot sizefor processing the metal material layer and a larger spot size forprocessing the dielectric material layer.
 17. The laser system of claim12 in which the dielectric material layer comprises an organicdielectric material and the metal material layer comprises copper. 18.The laser system of claim 12 in which the central irradiance profile hasa profile size when it reaches the aperture, the aperture has anaperture size that influences the percentage of Gaussian energy thatreaches the target, and the zoom lens assembly is adjusted to matchsubstantially the size of the profile size with the aperture size.
 19. Alaser system, comprising: a diode-pumped, solid-state laser forgenerating Gaussian laser output having a Gaussian energy along anoptical path; a first removable imaging optics rail positioned along theoptical path, the first imaging optics rail housing a first beam shapingcomponent and a first aperture, the first beam shaping component beingpositioned along a first rail path collinear with the optical path toconvert the Gaussian laser output to first shaped output having a firstcentral irradiance profile of high and uniform intensity and a firstouter irradiance profile of low intensity, the first aperture beingpositioned along the optical path to clip a first portion of the firstouter irradiance profile of the first shaped output and pass at least50% of the Gaussian energy through the first aperture to produce firstapertured shaped output having first apertured shaped energy, the firstimaging optics rail being replaceable with a second imaging optics railhousing a second beam shaping component and a second aperture, thesecond beam shaping component being positioned along a second rail pathdesigned to be collinear with the optical path to convert the Gaussianlaser output to second shaped output having a second central irradianceprofile of high and uniform intensity and a second outer irradianceprofile of low intensity, the second aperture being positioned along theoptical path to clip a second portion of the second outer irradianceprofile of the second shaped output and pass at least 50% of theGaussian energy through the second aperture to produce second aperturedshaped output having second apertured shaped energy, wherein the secondapertured shaped energy is of an amount different from that of the firstapertured shaped energy; one or more imaging lens components forconverting the first or second apertured shaped output into respectivefirst or second image shaped output, wherein the first image shapedoutput has a first irradiance on a work piece and the second imagedshaped output has a second irradiance on the work piece different fromthe that of the first irradiance profile; and a positioning system fordirecting the imaged shaped output toward a target location on the workpiece.
 20. The laser system of claim 19, further comprising: a pair ofbeam directing mirrors positioned along the optical path, thediffractive optical element and the aperture being positioned opticallybetween the beam directing mirrors, for diverting the Gaussian laseroutput along an alternative optical path that avoids the diffractiveoptical element and the aperture such that the beam positioning systemdirects the Gaussian output toward the work piece.
 21. The laser systemof claim 19 in which the diffractive optical element is a firstdiffractive optical element, the aperture is a first aperture having afirst size, and the first diffractive optical element and the firstaperture cooperate to produce a substantially uniform first energydensity over a first spot area on the work piece.
 22. The laser systemof claim 21 in which the first diffractive optical element and the firstaperture are removable and replaceable by a second diffractive opticalelement and a second aperture which cooperate to determine a secondsubstantially uniform energy density over a second spot area on the workpiece.
 23. The laser system of claim 21, further comprising: a firstremovable imaging optics rail that houses the first diffractive opticalelement and the first aperture, the first imaging optics rail beingreplaceable with a second imaging optics rail having a seconddiffractive optical element and a second aperture which cooperate todetermine a second energy density over a second spot area at the targetlocation on the work piece.
 24. The laser system of claim 19, furthercomprising: a removable imaging optics rail that houses the diffractiveoptical element and the aperture, such that removal of the imagingoptics rail permits the Gaussian laser output to impinge the work pieceat the target location.
 25. The laser system of claim 19 in which theapertured shaped energy is greater than 65% of the Gaussian energy. 26.The laser system of claim 25 in which the apertured shaped energy isgreater than 75% of the Gaussian energy.
 27. The laser system of claim19 in which the via has a minimum diameter d_(min), a maximum diameterd_(max), and a roundness of greater than 0.9, where the roundness equalsd_(min)/d_(max).
 28. The laser system of claim 19 in which the via has abottom diameter d_(b), a top diameter d_(t), and a taper ratio ofgreater than 0.5, where the taper ratio equals d_(b)/d_(t).
 29. Thelaser system of claim 19 in which the first energy density is less thanor equal to about 2 J/cm².
 30. The laser system of claim 19 in which thewavelength is about 355 nm or about 266 nm.
 31. The laser system ofclaim 19 in which the via comprises first and second layer materials,and the first layer material comprises a dielectric material and thesecond layer material comprises a metal.
 32. The laser system of claim31 in which the dielectric material comprises an organic dielectricmaterial and the metal comprises copper.
 33. The laser system of claim19 further comprising a variable beam expander positioned along theoptical path between the aperture and the work piece.
 34. The lasersystem of claim 19 further comprising rapidly variable controlelectronics for rapidly changing control of a Q-switch for effecting arapid repetition rate change in the Gaussian laser output to convert itto a second Gaussian laser output having a second pulse energy.
 35. Amethod for substantially matching the performance of substantiallysimilar first and second laser systems, the first laser system having afirst diode-pumped, solid-state laser for generating first ultravioletGaussian laser output having a first Gaussian energy along a firstoptical path; a first diffractive optical element positioned along thefirst optical path for converting the first Gaussian laser output tofirst shaped output having a first central irradiance profile of highand uniform intensity and a first outer irradiance profile of lowintensity, a first variable beam expansion assembly positioned along thefirst optical path between the first laser and the first diffractiveoptical element, a first aperture positioned along the first opticalpath for clipping a major portion of the first outer irradiance profileof the first shaped output and passing at least 50% of the firstGaussian energy through the first aperture to produce first aperturedshaped output having first apertured shaped energy, one or more firstimaging lens components for converting the first apertured shaped outputinto first image shaped output; and a first positioning system fordirecting the first imaged shaped output toward a first target locationon a first work piece, the second laser system having a seconddiode-pumped, solid-state laser for generating second ultravioletGaussian laser output having a second Gaussian energy along a secondoptical path; a second diffractive optical element positioned along thesecond optical path for converting the second Gaussian laser output tosecond shaped output having a second central irradiance profile of highand uniform intensity and a second outer irradiance profile of lowintensity, a second variable beam expansion assembly positioned alongthe second optical path between the second laser and the seconddiffractive optical element, a second aperture positioned along thesecond optical path for clipping a major portion of the second outerirradiance profile of the second shaped output and passing at least 50%of the second Gaussian energy through the second aperture to producesecond apertured shaped output having second apertured shaped energy,one or more second imaging lens components for converting the secondapertured shaped output into second image shaped output; and a secondpositioning system for directing the second imaged shaped output towarda second target location on a second work piece, the first and secondlasers or the first and second diffractive optical elements haveunintentional performance differences, comprising: impinging the firsttarget with the first imaged shaped output of the first laser system;impinging the second target with the second imaged shaped output of thesecond laser system; comparing first results of impingement with thefirst imaged shaped output to second results of impingement with thesecond imaged shaped output; and employing the first and/or secondvariable beam expansion assemblies to compensate for performancedifferences between the first and second results such that the first andsecond laser systems exhibit substantially the same performance for agiven laser application.
 36. A method for substantially matching asecond performance of a laser system on a second target of a secondmaterial to a first performance of the laser system on a first target ofa first material, the laser system including a diode-pumped, solid-statelaser for generating ultraviolet Gaussian laser output having a Gaussianenergy along an optical path, a beam-shaping element positioned alongthe optical path for converting the Gaussian laser output to shapedoutput having a central irradiance profile of high and uniform intensityand an outer irradiance profile of low intensity, an aperture positionedalong the optical path for clipping a major portion of the outerirradiance profile of the shaped output and passing a major portion ofthe Gaussian energy through the aperture to produce apertured shapedoutput having apertured shaped energy, a zoom lens assembly positionedalong the optical path between the aperture and a target location on awork piece, one or more imaging lens components for converting theapertured shaped output into image shaped output, and a positioningsystem for directing the imaged shaped output toward the target locationto form a via, the first and second target materials having differentlaser ablation characteristics such that a given set of laser parameterswould form with a first imaged shaped output a first via of a firstmajor spatial dimension in the first target material and the same set oflaser parameters would form with a second imaged shaped output a secondvia of second major spatial dimension in the second target material,comprising: impinging the first target material with the first imagedshaped output of the first spot size to form the first via of the firstmajor spatial dimension; impinging the second target with the secondimaged shaped output of the second spot size to form the second via ofthe second major spatial dimension; comparing the first and secondspatial dimensions; and employing the zoom lens assembly to compensatefor differences between the first and second major spatial dimensionssuch that vias formed in the first and second target materials exhibitsubstantially similar major spatial dimensions with substantiallysimilar laser parameters.